INFLUENCE OF CLAY CONTENT ON RESIDUAL LIQUEFACTION IN WAVES
V.S. Ozgur Kirca, Istanbul Technical University, kircave@itu.edu.tr
B.Mutlu Sumer, Technical University of Denmark, bms@mek.dtu.dk
Jørgen Fredsøe, Technical University of Denmark, jf@mek.dtu.dk
INTRODUCTION
Seabed soil may undergo residual liquefaction under the
action of water waves where the effective stresses
between the individual grains vanish, and therefore the
water and sediment mixture acts like a liquid. This
phenomenon has been studied quite extensively,
especially in the last two decades (see Sumer, 2014 for a
detailed review). In those studies on wave-induced
residual liquefaction, the seabed has been considered to
contain one type of soil such as silt, or sand. However, it
is not uncommon that the seabed contains clay, and
therefore the seabed soil in these cases behaves as a
composite soil such as, for example, clayey silt or clayey
sand. This study presents the results of an experimental
investigation of the influence of clay content on residual
liquefaction of seabed beneath regular progressive waves.
An elaborate journal paper on the subject has recently
been accepted for publication, which can be consulted for
details (Kirca et al., 2014).
EXPERIMENTS
Experiments were conducted in a 26.5 m x 0.6 m wave
flume, specifically constructed for liquefaction tests. In the
middle of the flume length, a 0.4 m deep and 0.78 m long
soil pit was installed beneath the bottom, such that the
top level of the soil in the pit became flush with the flume
bed (Kirca et al., 2014). Water-surface elevation
measurements were conducted on the flume as well as
pore-water pressure measurements across the soil depth
at five points. These measurements were synchronized
with video recording of the liquefaction process from the
side. The range of tested wave heights was H = 7.6 – 18.3
cm, whereas a constant wave period T = 1.6 s, and water
depth h = 55 cm. was used. Experiments were, for the
most part, made with silt (d50 = 0.07 mm) and silt-clay
mixtures, which were complemented with some tests
made with sand-clay mixtures. The clay used was the
blue clay, commercially available in Denmark.
RESULTS
The experiments showed that the influence of clay
content on wave induced liquefaction is very significant.
Susceptibility of silt to liquefaction (i.e. the rate of porewater pressure buildup) is increased with increasing clay
content, CC, up to CC ≈ 30% beyond which the mixture of
silt and clay is not liquefied (Figure 1). It may be noted
that the latter limit, CC ≈ 30%, is clay specific.
Furthermore, it was seen that when clay is added, the
duration during which the soil stayed in liquefied state is
significantly extended (Figure 2). In the experiments with
sand-clay mixtures, three different sands were used,
namely d50 = 0.17, 0.4 and 0.9 mm. These tests show that
sand may become prone to liquefaction with the
introduction of clay, contrary to the general perception
that this type of sediment is normally liquefaction resistant
under waves. For instance, sand with d50 = 0.4 mm was
liquefied with a clay content of CC = 10.8% while sand
with d50 = 0.17 mm was partially liquefied with a clay
content as small as CC = 2.9%.
Finally remarks are made as to how to check for
liquefaction potential of clayey soils exposed to waves in
real life situations.
Figure 1 – Number of waves for pressure to reach its
maximum value at soil depth 39 cm for H =18.3 cm.
Figure 2 – Time during which the soil remains in the
liquefaction state at soil depth 39 cm for H = 18.3 cm.
ACKNOWLEDGEMENT
We acknowledge the support from the EU commission,
through the FP-7 project MERMAID (G.A. No. 288710).
REFERENCES
Kirca, Sumer, Fredsøe (2014): Influence of Clay Content
on Wave-Induced Liquefaction, J. of Waterway, ASCE
(accepted for publication).
Sumer (2014): Liquefaction around Marine Structures,
World Scientific, Singapore, 472 pp.